Fix a data race for NNUE

[stockfish] / src / thread.cpp

diff --git a/src/thread.cpp b/src/thread.cpp
index 44aea14e6e0aab0a2d11d03ce9f4817a3fe12fff..1aa66a816619549c60f76f38cf8b030e0828a48d 100644 (file)
--- a/src/thread.cpp
+++ b/src/thread.cpp
@@ -204,21 +204,18 @@ void ThreadPool::start_thinking(Position& pos, StateListPtr& states,
 
   // We use Position::set() to set root position across threads. But there are
   // some StateInfo fields (previous, pliesFromNull, capturedPiece) that cannot
-  // be deduced from a fen string, so set() clears them and to not lose the info
-  // we need to backup and later restore setupStates->back(). Note that setupStates
-  // is shared by threads but is accessed in read-only mode.
-  StateInfo tmp = setupStates->back();
-
+  // be deduced from a fen string, so set() clears them and they are set from
+  // setupStates->back() later. The rootState is per thread, earlier states are shared
+  // since they are read-only.
   for (Thread* th : *this)
   {
       th->nodes = th->tbHits = th->nmpMinPly = th->bestMoveChanges = 0;
       th->rootDepth = th->completedDepth = 0;
       th->rootMoves = rootMoves;
-      th->rootPos.set(pos.fen(), pos.is_chess960(), &setupStates->back(), th);
+      th->rootPos.set(pos.fen(), pos.is_chess960(), &th->rootState, th);
+      th->rootState = setupStates->back();
   }
 
-  setupStates->back() = tmp;
-
   main()->start_searching();
 }